The present invention relates to a treatment of melt in a recovery boiler so as to reduce the amount of sulphur compounds entering the recovery boiler and thus to minimize the amount of malodorous gases at the mill, the sulphur emissions from the boiler, and corrosion problems in the boiler.
In the manufacture of cellulose chemical pulp in the pulp and paper industry produced waste liquor, typically referred to as "black liquor", is normally combusted in a boiler to recover chemicals and heat. In a conventional recovery boiler the process chemicals are recovered by injecting the liquor to the boiler, whereby it dries quickly and burns under reducing conditions, generating a melt. This melt mainly contains sodium carbonate, sodium sulphide and sodium sulphate. A number of other minor compounds are present, but the essential components in the manufacture of cellulose pulp are Na.sub.2 CO.sub.3 and Na.sub.2 S.
A large volume and variety of sulphurous gases are generated in a sulphate mill, which are combusted either in a recovery boiler or in a lime sludge reburning kiln. Particularly it is possible to generate a large volume of sulphurous gases by pressure heating black liquor, whereby a gas containing mercaptan, dimethyl sulphide, hydrogen sulphide and other components is generated. Thus, a part of sulphur may be separated from black liquor prior to the liquor being introduced to the recovery boiler, and thus the sulphur loading and the corrosion problems of the recovery boiler are reduced. However, the reduced sulphur loading also reduces the sulphur content of the melt formed in the boiler, and thus also that of the cooking liquor produced from the melt, to an unacceptably low level (i.e. the sulphur content is too low to reach the high sulphidity required in pulp cooking).
By the present invention, the sulphur content of the melt may be raised to a desired level in a simple manner. The most advantageous way to raise the sulphur content of the melt is to utilize the sulphurous off gases of the pulp mill.
In accordance with one aspect of the present invention, a method of adjusting one or both of the sulphur content and sulphur compound composition of a melt formed in a pulp mill boiler (recovery boiler) is provided. The method comprises the step of bringing at least a portion of the boiler melt into contact with a gas selected from the group consisting essentially of gaseous sulphur compounds and a reducing gas. The method may be practiced by treating a sulphurous pulp mill gas to form the gas utilized in the method, and then reacting the gas with a melt to produce an increased sodium sulphide content and an exhaust gas. The exhaust gas and melt are separated, and the exhaust gas can be used to provide heat to facilitate the reaction between the melt and the sulphur-containing or reducing gas, or it can be burned in a boiler or kiln, while the increased sodium sulphide content melt is further treated, as by dissolving it to produce green liquor, which ultimately (either by causticization or directly) can be used as a cooking liquor in the production of chemical cellulose pulp.
According to another aspect of the present invention, an apparatus for the treatment of a melt is provided. The apparatus comprises the following elements: A recovery boiler, a melt produced in a bottom portion thereof. Means defining a reaction space in communication with the recovery boiler for receiving the melt produced therein, for carrying out reactions between the melt and a gas selected from the group consisting essentially of gaseous sulphur compounds and a reducing gas. Means for supplying melt to the reaction space. Means for supplying a gas selected from the group consisting essentially of gaseous sulphur compounds and a reducing gas to the reaction space. Means for discharging reacted gas from the reaction space. And, means for discharging melt from the reaction space.
Typically the means for supplying gas to the reaction space comprises a ceramic pipe, e.g. with a nozzle at the end, for bubbling gas into the melt below the surface of the melt.
According to another aspect of the present invention, a method of reducing the sulphur emissions from, and corrosion in, a recovery boiler of a pulp mill is provided. The method comprises the steps of: (a) Pressure heating black liquor to produce sulphur-containing off gases, and to reduce the sulphur content of the black liquor. (b) Burning the black liquor in the recovery boiler to produce a melt having a lower sulphidity than if step (a) were not practiced. And, (c) bringing at least a portion of the melt produced in step (b) into contact with the sulphurous off gases from step (a) to thereby increase the sulphidity of the melt so that it has a sulphidity sufficient to be utilized as primary cooking liquor in the production of chemical cellulose pulp.
The present invention is preferably practiced by using gas containing dimethyl sulphide generated in the pressure heating of black liquor, however, the invention is not restricted to that source of gas; rather all possible sulphurous gases in a pulp mill may be employed, for example, gases from the digester house.
It is the primary object of the present invention to increase the sulphidity of a recovery boiler melt, and/or to reduce the sulphur emissions from and corrosion within a recovery boiler. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.